1. Field of the Invention
This invention relates to a semiconductor nonvolatile memory, to a method of recording data into the semiconductor nonvolatile memory (writing method and erasing method) and to a method of reading data from the semiconductor nonvolatile memory.
2. Description of the Related Art
Owing to its capability of holding stored data without requiring electric power, the semiconductor nonvolatile memory has been utilized as a memory in low-power-consumption equipment such as portable equipment.
In recent years, there has been proposed, as a semiconductor nonvolatile memory, a structure equipped with a memory cell of the MONOS (metal oxide nitride oxide) type having at least two gate electrodes (see, for example, U.S. Pat. No. 5,408,115 and U.S. Pat. No. 6,399,441).
The memory cells of the MONOS type are of the constitution including a transistor having a general gate-insulating film and a transistor having a gate-insulating film comprising an ONO laminated insulating film capable of accumulating an electric charge, which are formed on the channel-forming regions.
In the semiconductor nonvolatile memories disclosed in the above U.S. Pat. No. 5,408,115 and U.S. Pat. No. 6,399,441, however, the channel concentrations must be separately optimized in the channel-forming regions that are formed under the ONO laminated insulating film and under the gate-insulating film. This, however, involves complex operation when it is attempted to operate the memory and, besides, makes it difficult to easily and efficiently inject the electric charge into the ONO laminated insulating film.
Further, the memory cell structure becomes complex since it has at least two gate electrodes as well as a gate-insulating film that includes the ONO laminated insulating film, and the cost of production increases.
In order to decrease the area occupied by the memory cell, therefore, there has been proposed a structure in which the gate electrode on the ONO laminated insulating film is formed in the side wall type. When a voltage is applied, however, the electrode of the side wall type exhibits an increased resistance bringing about such a hindrance that the speed for reading out the data becomes low.